The invention relates to gas lasers, and in particular to adjustability of the output power of a helium-neon laser.
Helium-neon lasers have not been sufficiently adjustable in output power for many application. Variation of certain electric parameters, such as changing the current in the tube, can be used to adjust the power, but there are problems associated with this approach.
The principal problem with modulating beam power through variation of the current in the plasma tube has been that when the current is reduced in the helium-neon tube, the plasma begins to flicker, as if going on and off. The tube becomes unstable, with very high noise, and it becomes difficult to produce a stable output beam.
Various attempts have been made previously to overcome this problem. One method was to raise the gas pressure in the tube, and run the tube at much higher currents, causing a loss of efficiency. Also, optical noise problems can result with this approach.
The lasing phenomenon in a helium-neon tube is not very fast. Once the plasma is ionized, the photons must bounce back and forth between the cavity end mirrors to establish lasing. Several hundred bounces are required before there is any significant lasing. This limits the speed at which the laser can be modulated by directly modulating the plasma.
It appears that what happens at low current levels in the tube is that the density of electrons in the vicinity of the cathode becomes sparse and unstable, so that the cathode intermittently goes out and there is a failure of the plasma to conduct until the voltage build-ups.
Many important uses could be made of a helium-neon laser if the power could be regulated by changing the current in the tube, achieving modulation of the tube power to a reasonable depth. It is an object of the present invention to provide a gas laser tube capable of such modulation.